Known soft magnetic materials for magnetic cores or the like include metal soft magnetic materials such as Sendust and Permalloy and metal oxide soft magnetic materials such as ferrite.
The metal soft magnetic materials have a high saturation magnetic flux density and high magnetic permeability, but experience great eddy current losses in a high frequency band because of low electric resistivity. They are thus difficult to use in the high frequency band.
In turn, the metal oxide soft magnetic materials provide less eddy current losses in the high frequency band because of their higher electric resistivity than the metal soft magnetic materials. However, the metal oxide soft magnetic materials are unsatisfactory in saturation magnetic flux density.
Under such circumstances, composite soft magnetic materials having high saturation magnetic flux density and magnetic permeability as well as high electric resistivity were proposed as the soft magnetic material which overcame the drawbacks of both the metal soft magnetic material and the metal oxide soft magnetic material.
For example, Japanese Patent Application Kokai No. 91397/1978 discloses a high magnetic permeability material comprising a metal magnetic material having a coating of high magnetic permeability metal oxide formed on the surface;
Japanese Patent Application Kokai No. 164753/1983 discloses a composite magnetic material prepared by mixing an oxide magnetic material powder and a metal magnetic material powder composed of an Fe-Ni base alloy and molding the mixture; and
Japanese Patent Application Kokai No. 13705/1989 discloses a composite magnetic material having a saturation magnetic flux density Bs of 6.5 to 20 kG, comprising a soft magnetic metal magnetic powder having a mean particle size of 1 to 5 .mu.m and a soft magnetic ferrite wherein the soft magnetic ferrite fills in between the metal magnetic powder particles so that the metal magnetic powder particles are independent from each other while the soft magnetic ferrite portion is continuous.
Prior art composite soft magnetic materials including the ones disclosed in the foregoing patent publications are fired by hot press sintering, vacuum sintering, and atmospheric pressure sintering processes like atmosphere sintering. The firing temperature generally ranges from about 900.degree. to about 1200.degree. C. and a firing time of one hour or longer is generally required.
However, metal soft magnetic materials, when held for more than one hour at elevated temperatures, are oxidized by oxygen available from the metal oxide soft magnetic materials which are, in turn, reduced. The situation remains the same even when the materials are fired in a reducing atmosphere.
Since the metal soft magnetic material and metal oxide soft magnetic material lose their own features, a composite soft magnetic material having high saturation magnetic flux density and magnetic permeability as well as high electric resistivity is no longer obtained.